Okay, let me start with a fact you might not know. It is impossible for any human (as far as I know there are no exceptions) to drink a gallon of milk without vomiting. My brother and his friends proved this on several occasions in college when they had contests to see who could keep it down the longest (I believe he won but inevitably they all through up).
That being said, it sounds like she really wanted to suck and possibly felt hungry. I would try a serving of cereal around 7 pm. See if this slows the formula binge down. I really think that it was probaby the amount of formula she had in a short period of time.
That being said, you still want to watch how she reacts tot he new food and see if she does it again without the extra formula. However, vomiting is not usually a symptom of an allergy. That doesn't mean that it never is but it usually isn't. Also, there is usually more than just one symptom. You would probably see some stomache upset earlier than you did, some congestion or a rash. Like I said, usually. That doesn't mean that it wasn't. But given the milk/formula intake, I would venture to say it was probably the large amount of formula in a short time. My son has done this to me a couple of times (He's now 15 1/2 months old). Sometimes, it during a growth spurt they really think they need that extra formula. We love our babies and give them what they want and occasionally, it is too much.
Relax, don't worry yet. Just keep track of it so that if it happens again, you can share it with her pediatrician to be sure everything is fine. At this point, to me, it sounds like to much of a good thing.
Best of luck with your little one.